Automobile engines have been produced with various numbers of cylinders over the years (from 2 to 16), but the most common engines in modem cars are in-line four-cylinder designs for smaller cars, and V-6 and V-8 designs for larger cars. All V-8 engines have an angle of 90 degrees between the two banks of four cylinders, whereas the V-6 engines have an angle of either 60 degrees or 90 degrees between the two banks of three cylinders. Smaller engines, in terms of both cylinder number and displacement, provide improved fuel economy, which is particularly important because of rigid federal corporate average fuel economy (CAFE) standards. Neither four- nor six-cylinder engines operate as smoothly as a V-8 because of less effective dynamic balancing and greater torque fluctuations.
Nearly every internal combustion engine includes a muffler. A wide variety of muffler designs have been employed in the prior art. Internal combustion engines operating under full loads have quickly opening exhaust valves that release quick surges of exhaust gas under substantial residual pressure. For example, the pressure of an exhaust gas surge may be as high as 2.7 atmospheres. In an automobile engine, the engine releases large periodic amounts or surges of pressurized gas. It is common knowledge that, if not muffled, such sudden releases of pressurized gas produce considerable objectionable noises or compressional sound waves having various frequency components. In the case of a four cylinder, four-stroke cycle engine, the exhaust gas surges are produced once every revolution. Thus, in the example of a four cylinder, four-stroke cycle engine operating at 3000 R.P.M., or fifty revolutions per second, fifty exhaust gas surges are produced per second which has a high likelihood of causing objectional noises or compressional sound waves.
Exhaust systems have several purposes, one of which is to direct the output gases from the engine to a position near the periphery of the automobile. A muffler is used in the exhaust gas system to silence the objectional noises or compressional sound waves caused by the periodic gas surges output from the engine. In order to be efficient as sound silencers, exhaust mufflers must decrease the exhaust gas velocity and either absorb the sound waves or cancel them by interference with other waves form the same source. Mufflers should have volumes 6 to 8 times the piston displacement and may contain baffles with or without holes. Mufflers that cancel sound waves by interference usually break the waves into two parts which follow different paths and meet again, out of phase, before leaving the muffler.
Exhaust gas back pressure should be kept to a minimum since an increase of 1 lb/in.sup.2 (6.9 Kpa) in back pressure decreases the maximum power output about 2 percent, about 1 percent being due to more exhaust work and the balance of the effect of increased clearance gas pressure on volumetric efficiency.
In addition to objectionable noise, exhaust gas from an internal combustion engine generally includes a variety of constituents that are considered to be pollutants. Such exhaust gas constituents include unburned hydrocarbons and carbon monoxide (CO). Engine exhaust systems typically include a catalytic converter for treating the exhaust gas by promoting more complete oxidation of the exhaust gas constituents, by changing unburned hydrocarbons to carbon dioxide (CO.sub.2) and water, and changing carbon monoxide (CO) to carbon dioxide (CO.sub.2).
Mufflers today have a variety of problems. Among the problems are that the construction of a muffler using baffles or multiple gas paths is generally fairly expensive. An alternative that could be built for less money would be attractive.